This application is based upon and claims the benefit of Japanese Patent Applications No. 9-199972 filed on Jul. 25, 1997, and No. 10-157915 filed on Jun. 5, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a pressure detecting apparatus suitable for detecting a high pressure of fluid, particularly to a pressure detecting apparatus utilizing a piezo resistance effect of single-crystal semiconductor.
2. Description of the Related Art
JP-B2-7-11461 discloses such a pressure detecting apparatus, which has a metallic diaphragm integrally formed with a sensing body and a square sensor chip (semiconductor chip) holding strain gauge resistors thereon and bonded to the diaphragm through a glass layer. In the pressure detecting apparatus, thermal stress is produced due to a difference in linear thermal expansion coefficient between the semiconductor chip and the metallic diaphragm, and it is applied to the sensor chip to cause a detection error. To reduce such a detection error, conventionally, the following countermeasures are proposed. That is, the metallic diaphragm is made of material having a linear thermal expansion coefficient close to that of the sensor chip, and the metallic diaphragm and the sensor chip are thinned to improve sensitivity, i.e., to relatively reduce the detection error caused by the thermal stress. However, it is difficult to sufficiently reduce the detection error only by the above countermeasures.
It is an object of the present invention to provide a pressure detecting apparatus including a semiconductor sensor chip bonded to a metallic diaphragm and capable of precisely detecting a high pressure with simple structure. Another object of the present invention is to provide a pressure detecting apparatus capable of minimizing an adverse effect to a detection error by a thermal stress that is produced due to a difference in linear thermal expansion coefficient between a metallic diaphragm and a sensor chip.
Briefly, according to the present invention, a single-crystal semiconductor sensor chip disposed on a metallic diaphragm has a planar shape selected from a group consisting of a circular shape, a first polygonal shape having more than five sides and having interior angles all less than 180xc2x0, and a second polygonal shape having a ratio of a circumscribed circle diameter relative to an inscribed circle diameter being less than 1.2.
In this state, when the sensor chip is made of a single-crystal semiconductor having an approximately (100) crystal orientation, a strain gauge resistors can be disposed point-symmetrically with respect to a center point of the sensor chip on first and second axes passing through the center point in parallel with  less than 110 greater than  directions of the single-crystal semiconductor. When the sensor chip is made of a single-crystal semiconductor having an approximately (110) crystal orientation, one of the plurality of strain gauge resistors is disposed at a central portion of the sensor chip, while another of the plurality of strain gauge resistors is disposed at a peripheral portion of the sensor chip. Accordingly, an adverse effect of a thermal stress caused by a difference in linear thermal expansion coefficient can be reduced with simple structure, resulting in decrease in a detection error. At the same time, a rate of change in resistance of the strain gauge resistor increases, resulting in high sensitivity.
Instead of the sensor chip, a bonding member interposed between the sensor chip and the diaphragm may have a planar shape selected from a group consisting of a circular shape, a first polygonal shape having more than five sides and having interior angles all less than 180xc2x0, and a second polygonal shape having a ratio of a circumscribed circle diameter relative to an inscribed circle diameter being less than 1.2. Accordingly, the same effects as described above can be provided.
When the sensor chip made of a single-crystal semiconductor has a rectangular shape with first and second sides, in stead of controlling the shape of the bonding member, one of the first and second sides is set to form a specific angle in a range of 15xc2x0 to 37xc2x0 with one of crystal directions perpendicular to one another and parallel to a surface plane of the sensor chip. More preferably, the specific angle of the one of the first and second sides of the sensor chip and the one of the crystal directions is 15xc2x0 to 33xc2x0. A plurality of strain gauge resistors can be disposed on first and second axes passing through the center point in parallel with the crystal directions perpendicular to one another. Accordingly, the same effect as described above can be provided.
When the single-crystal semiconductor forming the rectangular sensor chip has an approximately (100) plane orientation, the crystal directions correspond to  less than 110 greater than  directions of the single-crystal semiconductor. In this case, specific angle xcfx86 between one of the sides of the sensor chip and one of the  less than 110 greater than  directions is expressed by the following equation:
xcfx86=Axe2x88x92Bec(T+t)
where T represents a thickness of the diaphragm, t represents a thickness of the bonding member, and A, B, C represent predetermined constants, respectively.
Preferably, T is a range of 100 xcexcm to 3500 xcexcm, t is in a range of 15 xcexcm to 150 xcexcm, A is in a range of 40 to 45, B is in a range of 20 to 30, and C is in a range of xe2x88x924xc3x9710xe2x88x924 to xe2x88x922xc3x9710xe2x88x924. Accordingly, the pressure detecting apparatus of the present invention can be readily designed to reduce the detection error and to provide high sensitivity.